1. Technical Field
The present invention relates generally to a lithium ion secondary battery, and more particularly to a lithium ion secondary battery using a negative electrode material with its capacity reduction held back.
2. Related Art
Lithium ion secondary batteries with a lithium containing composite oxide as a positive electrode and a carbon material or lithium metal as a negative electrode, because of achieving high energy densities, have now attracted attention as power sources for cellular phones, notebook PCs, etc. They have also received attention as power sources for hybrid cars, because of making sure high input/output performance.
Generally for a typical lithium ion secondary battery, the negative electrode comprises a negative electrode active material layer in which a negative electrode active material composed mainly of carbon is mixed with an electrical conducting material for making sure electrical conductivity in the active material layer and a binder for making sure inter-particle adhesion.
For instance, Japanese Patent No. 3239302 discloses a negative electrode in which carbon of low graphitization (a carbon material including an amorphous moiety and a crystalline moiety) is mixed with finely divided carbon particles and/or fine fibrous graphite, and describes that, by using that, losses of charge/discharge capacity are reduced with improvements in charge/discharge cycle performance.
JP-A-2002-231316 puts forward a negative electrode comprising a mixture of a carbon material of low crystallinity with 2 to 6% by mass of an electrical conducting material higher in electrical conductivity than that carbon material of low crystallinity, and alleges that, by using that, the internal resistance of the battery is kept against increasing with improvements in the output performance. With the technique of this prior publication, however, the use of the electrical conducting material causes a drop of the ratio of the active material in the negative electrode, which otherwise brings about a lowering of the negative electrode capacity.
JP-A-2002-334693 shows a negative electrode comprising a mixture of a carbon material having a latice spacing of 0.335 to 0.410 nm with 5 to 30% of vapor-grown carbon fibers, and alleges that, by using that, electrode reactions proceeds so smoothly that there are improvements in charge/discharge cycle performance, etc.
However, the vapor-grown carbon fibers described there as acting as a combined active material and electrical conducting material has a high degree of graphitization given in terms of the (002) lattice spacing of 0.336 to 0.337 nm: it is substantially made up of a carbon material/graphite mixture that may work for prevention of a lowering of the negative electrode capacity. However, the mixing ratio must be brought up to as high as 5 to 30% to make sure electrical conductivity, which may have a risk of incurring side effects having adverse influences on battery performance, for instance, the breakdown of an electrolysis solution due to its reaction with graphite.